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o or o n a y
LED Package Reliability
(C l P i t St bilit ) Ralph C. Tuttle
Cree, Inc [email protected]
(Color Point Stability)
Color Bins Shown
CIE 1931 Color Space
ANSI C78.377 Color Bins Shown
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CIE 1931 Color Space
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CIE 1931 Color Space
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CIE 1931 Color Space
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Observed Color Shift with LEDs
Green Shift Yellow Shift
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Blue Shift
Color Shift with Plastic Packages
The white material in the package cavity reflects much of the light emitted from the LED chips.
The overall chromaticity (color) of the LED is a combination of all of the light emitted directly from the chips through the phosphor and that which is reflected internally in the package.
Short distance through phosphor Long distance through phosphor = cooler color temperature = warmer color temperature
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Color Shift with Plastic Packages
Short distance through phosphor Long distance through phosphor = cooler color temperature = warmer color temperature
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Color Shift with Plastic Packages
These photons will continue to These photons will not be be emitted from the package reflected off the package walls.
Over time the materials in these packages will discolor.
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Color Shift with Plastic Packages
An overall blue shift and lumen depreciation results
These photons will continue to These photons will not be be emitted from the package reflected off the package walls.
Over time the materials in these packages will discolor.
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LM-80 Test – PPA Plastic Packages
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Tsp = 85ºC
LM-80 Test – PPA Plastic Packages
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Tsp = 85ºC
Storage Test – PPA Packages
LEDs placed un-powered in reliability test chamber.
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Storage Test – EMC Packages
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LM-80 Test – EMC Packages
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Storage Test – Ceramic Packages
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Blue Color Shift with Plastic Packages
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Phosphor Chip Coated LED
Phosphor Silicone Lens
LED Chip
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Anode Cathode
Wire Bond
Substrate
Phosphor Chip Coated LED (High Temp Operation)
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Phosphor Chip Coated LED
At high temperatures cracking and delamination of the phosphor can occur at the chips surface. The light scattering increase in the angle at which the emitted light travels through the phosphor. The path of the blue photons through the phosphor is increased causing a shift towards the warm end of the spectrum.
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High temperature LM-80 testing has been used to accelerate the color shift in high power LEDs.
125°C
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105°C
85°C
55°C
Yellow Color Shift
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Can models be developed to project color shift over time?
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Color Shift during WHTOL testing
60/90 WHTOL Testing
Tsp = 60ºC Tair = 60ºC 90% RH 90% RH 1 hour on 1 hour off
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Green Color Shift
Red Phosphor degrades causing a shift of the color point.
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im ortant.
Summary
• Color shift can happen with LEDs.
• There are many applications where it is not critical.
• There are many applications color point stability is important. p
• Materials used in LED package construction will result in different color point stability characteristics.
• There is no method currently available to project color shift.
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